1. Field of the Invention
The present invention relates to a polyester composition. In particular, the present invention relates to a polyester composition which exhibits an enhanced fiber-forming (melt-spinning, drawing) property, film-forming property, and molding property, and which is capable of being converted to various polyester fibers, film, and other shaped articles having enhanced mechanical properties. More particularly, the present invention relates to a polyester composition extremely useful for producing polyester filaments having excellent mechanical properties, for example, excellent tensile strength and Young's modulus, by means of a high speed melt-spinning method.
2. Description of the Prior Arts
Polyester resins, especially, polyethylene terephthalate resin, are useful for producing various filaments, fibers, films, and other shaped articles, because they exhibit excellent physical and chemical properties.
The polyester resins are produced usually in such a manner that an ester of an aromatic dicarboxylic acid with an alkylene glycol or its oligomer is produced by esterifying the aromatic dicarboxylic acid, especially, terephthalic acid, with the alkylene glycol, especially, ethylene glycol; by ester-interchanging an alkyl terephthalate with ethylene glycol; or by reacting terephthalic acid with ethylene oxide, and then subjecting the ester or its oligomer to a polycondensation-polymerization procedure to provide a polyester resin having a desired degree of polymerization.
The polyester resin produced by the above-mentioned process can be converted to shaped articles, for example, filaments or film, by melting the resin and extruding the melt through a spinneret having at least one orifice or a film-forming slit. If necessary, the extruded article is drawn at a predetermined draw ratio.
In a process for producing polyester filaments, a polyester resin melt may be extruded through a spinneret, solidified, and then wound up at a high speed of 2,000 m/min or more. The resultant intermediate oriented filaments are then draw-false-twisted, as disclosed in U.S. Pat. No. 3,771,307. This process is widely used in the polyester filament-producing industry.
Recently, U.S. Pat. No. 4,425,293 disclosed a process for producing polyester filaments, having a high level of physical properties sufficient for practical industrial use, only by a high speed melt spinning process at a high speed of 5000 m/min or more, without drawing the resultant melt-spun filaments.
However, in the high speed melt-spinning process, the high speed melt-spinning operation at a spinning rate of 5000 m/min or more causes a number of breakages of the individual filaments or filament bundle to increase, and thus the resultant filaments will contain broken individual filaments or fluffs and therefore, will exhibit a poor processability. The above-mentioned phenomenon becomes remarkable with a decrease in the denier of the individual filaments and with an increase in the number of the individual filaments in the melt spun filament bundle. Therefore, in practice, it is very difficult to carry out the industrial melt-spinning process at a very high winding speed of 6000 m/min or more.
In the filament-producing industry, however, it is strongly desired to provide, at a high speed, polyester filaments or fibers having high physical and mechanical properties, especially, a high tensile strength and an enhanced Young's modulus. These filaments are then highly susceptible to the finishing process, spinning process, and weaving or knitting process at a high productivity rate, and are useful for producing fiber or filament products having satisfactory functions and quality.
In the high speed melt-spinning process which is capable of producing polyester filament having satisfactory physical properties without drawing the melt-spun filaments, it is necessary to carry out the process at a very high winding speed of 5000 m/min or more. Even at the high speed of 5000 m/min or more, the resultant polyester filaments exhibit a tensile strength and Young's modulus lower than those of the filaments produced by a conventional melt-spinning-drawing process.
Accordingly, it is desired to provide a polyester resin which is susceptible to the high speed melt-spinning process and can be converted to polyester filaments having a satisfactory tensile strength and Young's modulus.
In the high speed melt-spinning process for polyester resins, it is known to disperse titanium oxide or silica in the polyester resin matrix. In this type of polyester composition, it was considered that the fine solid particles of the above-mentioned inorganic substances serves to enhance intermolecular slippage of the polyester molecular chains in the melt, to cause a rapid relaxation of the polyester molecule chains when the melt is deformed at a high speed, to prevent an undesirable concentration of stress at points at which the polyester molecular chains are entangled with each other, and which restrict orientation and crystallization of the polyester molecular chains. The above-mentioned effect of the solid fine particles is the so-called roller effect.
Generally, where fine solid particles are dispersed in a polymer matrix, the melt viscosity of the resultant polymer composition is increased. This phenomenon occurs because there is a viscosity-increasing effect in the slip plane between the fine solid particles and the polymer matrix, due to an interfacial formation of two electrical layers between the fine solid particles and the polymer matrix. Also, it is known that the increase in the melt viscosity becomes remarkable with a decrease in shearing speed applied to the melt of the polyester composition. This phenomenon occurs because, at a low shearing speed, the aggregation or network structure of the solid particles with each other and/or the solid particles with the polymer matrix is maintained stable, whereas, at a high shearing speed, the aggregation or network structure is ruptured, and therefore, the increase in the melt viscosity of the polyester composition derived from the solid fine particle becomes small.
In view of the above-mentioned phenomena, it was discovered by the inventors of the present invention that the fine solid particles of titanium dioxide or silica dispersed in the polyester resin matrix has no roller effect and restricts the intermolecular slip of polyester molecular chains. Also, the inventors discovered that the improvement in the high speed melt-spinning property of the polyester resin composition due to the solid fine particles of titanium dioxide or silica is mainly contributed to by another effect of the solid fine particle, that is, the restriction effect on orientation and crystallization of the polyester molecular chains.
However, it is very difficult to provide a polyester composition having very fine solid dispersoid dispersed in a polyester resin matrix by using conventional very fine solid particles. For example, when conventional fine solid particles, for example, colloidal silica or dry process silica particles, having a primary particle size of from 5 to 50 millimicrons are dispersed in a polymerization mixture for producing a matrix polyester resin and then the polymerization mixture is converted to a polyester composition at an elevated temperature, the fine solid particles are agglomerated by the action of the heat to form large secondary agglomerates having a size of 100 millimicrons or more. The large agglomerates are not effective for improving the melt viscosity property of the polyester composition.
The inventors of the present invention discovered that when 0.5 part by weight of tricalcium diphosphate (Ca.sub.3 (PO.sub.4).sub.2) are dispersed in 100 parts by weight of a polyethylene terephthalate resin matrix by reacting isopropyl acid phosphate with calcium acetate in at least one stage of procedures for producing the polyethylene terephthalate, the resultant polyester composition exhibits a remarkably increased melt viscosity at a low shearing rate.
Further, the inventors of the present invention discovered that the dispersed tricalcium diphosphate is in the form of a number of primary fine particles and a number of secondary agglomerates consisting of a plurality of the primary fine particle, the secondary agglomerates having a size of about 50 millimicrons or less and the increased entire surface area of the primary fine particles and secondary agglomerates contributes to the remarkable increase in the melt viscosity of the resultant polyester composition.
However, it was desired to provide a polyester composition wherein more fine solid particles are more stably, evenly dispersed in a polyester resin matrix than that described above.